1. Field of the Invention
The present invention relates to the field of switching power supplies, and particularly to a switching power supply using the resonance phenomenon of a capacitor and a coil.
2. Description of the Related Art
Since power supplies having high efficiency compared to switching power supplies of the related art can be obtained, synchronous rectifier type power supplies have recently been receiving attention.
Reference numeral 501 in FIG. 36 represents a synchronous rectifier type power supply of the related art. This power supply comprises a primary side bridge circuit 510, a secondary side rectification and smoothing circuit 520, a main transformer 530, and a control circuit 540.
The primary side bridge circuit 510 has four bridge transistors 511a, 511b, 512a and 512b (in this case, they are all n-channel MOSFETs).
The operation of the primary side bridge circuit 510 is divided into an A phase and a B phase, with bridge transistors that conduct during A phase operation being represented by reference numerals 511a and 512a, and bridge transistors that conduct during the B phase operation being represented by reference numerals 511b and 512b.
A primary winding 531 and a secondary winding 532 (532a, 532b) magnetically coupled to the primary winding 531 are provided inside the main transformer 530.
Both ends of the primary winding 531 are connected to the output section of the primary side bridge circuit 510, and the primary winding 531 and the four bridge transistors 511a, 511b, 512a and 512b are H-bridge connected.
Reference numeral 519 is a D.C. voltage source exemplified by a D.C. voltage obtained by rectifying and smoothing a commercial voltage, or a D.C. voltage output from a storage battery. The high voltage side of the D.C. voltage source 519 is connected to supply voltage line 517, while a low voltage side is connected to a ground line 518.
The primary side bridge circuit. 510 is connected to the supply voltage line 517 and the ground line 518. When the A phase bridge transistors 511a and 512a are turned on with the B phase bridge transistors 511b and 512b turned off, A phase current I.sub.A is supplied from the D.C. voltage source 519 to the primary winding 531.
On the other hand, when the B phase bridge transistors 511b and 512b are turned on with the A phase bridge transistors 511a and 512a turned off, B phase current I.sub.B. is supplied to the primary winding 531. The A phase current I.sub.A and the B phase current I.sub.B are opposite in direction to each other.
The secondary winding 532 has a terminal at its electrical center and an A phase secondary winding 532a and a B phase secondary winding 532b use the terminal as their common terminal (center tap).
The secondary side rectification and smoothing circuit 520 comprises a choke coil 525, an output capacitor 526 and two rectification transistors 523a and 523b.
The center tap of the A phase secondary winding 532a and the B phase secondary winding 532b is connected to a ground terminal 528, and the other terminals are connected to source terminals of respective rectification transistors 523a and 523b.
The drain terminals of both of the rectification transistors 523a and 523b are commonly connected to one terminal of the choke coil 525.
Reference numeral 527 represents the other end of the choke coil 525, and is connected to an output terminal. The output capacitor 526 is connected across the output terminal 527 and the ground terminal 528. Reference numeral 529 represents a load, which is also connected across output terminal 527 and the ground terminal.
The voltage on the output terminal 527 is isolated by a photocoupler 549 and input to the control circuit 540.
The control circuit 540 comprises a reference voltage source 541, a differential amplifier 542, an oscillator 543, a comparator 544, and a drive circuit 545. The differential amplifier 542 amplifies a difference between the voltage input from the photocoupler 549 and the output voltage of the reference voltage source 541, and supplies its output to the comparator 544.
The comparator 544 compares the voltage input from the differential amplifier 542 with the output waveform of the oscillator 543, and outputs the comparison result to the drive circuit 545.
The drive circuit 545 controls the time that the bridge transistors 511a, 512a, 511b and 512b are on so that a difference between the output voltage of the photocoupler 549 detected by the differential amplifier 542 and the output voltage of the reference voltage source 541 becomes small, based on the comparison result of the comparator 544.
Accordingly, even when the output voltage of the output terminal 527 fluctuates due, for example, to load variations, the primary side bridge circuit 510 is controlled by operation of the control circuit 540 so as to absorb these fluctuations, and the output voltage of the output terminal 527 is kept at a constant voltage.
Operation of the power supply 501 will now be described.
FIG. 37 shows the situation when the power supply 501 is operating, with the A phase and B phase bridge transistors 511a, 512a, 511b and 512b tuned off and current flowing in the secondary side due to energy stored in the choke coil 525.
Respective parasitic transistors 524a and 524b are formed inside the rectification transistors 523a and 523b. The parasitic diodes 524a and 524b are forward biased by electromotive force generated in the choke coil 525, and respective currents I.sub.551 and I.sub.552 flow.
FIG. 41 is a timing chart showing the operation of the power supply 501, and the above described state is represented as a waveform before time t.sub.1 in the timing chart.
From this state, a positive voltage is applied to the gate terminals of the A phase bridge transistors 511a and 512a, and when they are turned on, the two ends of the primary winding 531 are connected the supply voltage line 517 and the ground line 518. As a result, current represented by I.sub.553 in FIG. 38 flows.
The A phase secondary winding 532a is connected at a polarity to apply a positive voltage to a source terminal of the A phase rectification transistor 523a when the A phase bridge transistors 511a and 512a are on. In this state, a voltage of a polarity to apply a negative voltage to the source terminal of the B phase rectification transistor 523b is induced in the B phase secondary winding 532b.
The drive circuit 545 applies a positive voltage to the gate terminal of the A phase rectification transistor 523a and the gate electrodes of the A phase bridge transistors 511a and 512a.
In an n-channel MOSFET, when a voltage higher than the threshold voltage is applied to the gate terminal while the voltage on the source terminal is higher than the voltage on the drain terminal, current flows from the source terminal to the drain terminal in a direction that is the opposite of that for normal operation.
This operation is known as the third quadrant operation (in a p-channel MOSFET the condition where a voltage that is lower than the voltage on the drain terminal is applied to the source terminal and a voltage lower than the voltage on the drain terminal is applied to the gate terminal is called the third quadrant operation).
The solid line in FIG. 42 is a graph showing the characteristic of an n-channel MOSFET, with the horizontal axis representing drain terminal voltage V.sub.DS with reference to the source terminal, and the vertical axis representing drain current I.sub.D when a flow direction from the drain terminal to the source terminal is a positive direction.
The range in the first quadrant of this graph is normal MOSFET operation, and the solid line characteristic in the range of the third quadrant is the third quadrant operation. While the drain voltage V.sub.DS is small, a resistive characteristic is displayed, but as the drain voltage becomes large, when it becomes higher than a conducting voltage of the parasitic diode 524a, a diode characteristic is displayed.
The graph shown by the dotted line in the third quadrant of FIG. 42 is a parasitic diode characteristic when the MOSFET is not in a conducting state, and it will be understood that power loss becomes small during the third quadrant operation, compared to when current is flowing in the parasitic diode.
The A phase rectification transistor 523a has the internal parasitic diode 524a conducting and the source terminal voltage is higher than the drain terminal voltage.
In this state, since a positive voltage is applied to the gate terminal, the A phase rectification transistor 523a enters third quadrant operation and the current I.sub.554 in FIG. 38 flows in the direction from the source terminal to the drain terminal.
Accordingly, loss arising in the A phase rectification transistor 523a at this time is small.
Current I.sub.554 flowing in the A phase rectification transistor 523a is supplied through the choke coil 525 to the load 529 and the output capacitor 526, which means that magnetic energy is stored in the choke coil 525.
From this state, at time t.sub.2, when the A phase bridge transistors 511a and 512a and the A phase rectification transistor 523a are turned off, electromotive force is generated in the choke coil 525, and respective currents I.sub.555 and I.sub.556 flow in the two parasitic diodes 524a and 524b due to the energy stored in the choke coil 525, as shown in FIG. 38.
Next, at time t.sub.3, when the B phase bridge transistors 511b and 512b are turned on, current represented by I.sub.557 is supplied from the D.C. voltage source 519 to the primary winding 531, as shown in FIG. 40. At this time, a positive voltage is applied to the gate terminal of the B phase rectification transistor 523b, which means that the B phase rectification transistor 523b is in the third quadrant operation, current I.sub.558 flows in the choke coil 525 and energy is stored.
At time t.sub.4, when all of the bridge transistors 511a, 512a, 511b and 512b, and the rectification transistors 523a and 523b are turned off, the initially described state is returned to. After that, the above described operations are repeated.
As has been described above, transistors are used in the secondary side rectification circuit, the gate terminals are controlled to achieve reduced loss compared to the case where current flows due to third quadrant operation and diode elements are used in the rectification circuit.
However, when the transistors 511a, 512a, 511b, 512b, 523a and 523b are turned on, there is a recovery effect in the parasitic diodes 524a and 524b that causes a large current to flow momentarily, giving rise to the waveforms shown by reference numerals 561a, 562a, 561b and 562b in the timing chart of FIG. 41. When this current flows, there is a voltage applied across the source and drain, which causes loss.
In recent years, there has been increased demand to make power supplies highly efficient, and it has become impossible to ignore the problem of loss.